Effect of metal ion binding on the secondary structure of bovine .alpha.-lactalbumin as examined by infrared spectroscopy

• Published in: Biochemistry (Easton) 1991-09, Vol.30 (36), p.8797-8804
• Main Authors: Prestrelski, Steven J, Byler, D. Michael, Thompson, Marvin P
• Format: Article
• Language: English
• Subjects: ALBUMINAS; ALBUMINE; alpha -lactalbumin; Amides - chemistry; Animals; Biological and medical sciences; BOVIN; CALCIO; CALCIUM; Calcium - chemistry; CATION; CATIONES; Cattle; CINC; ESPECTROMETRIA; ESPECTROSCOPIA INFRARROJA; Fourier Analysis; Fundamental and applied biological sciences. Psychology; GANADO BOVINO; Lactalbumin - chemistry; MANGANESE; Manganese - chemistry; MANGANESO; METAL; METALES; Metals - chemistry; Metals - pharmacology; Molecular biophysics; Protein Binding - drug effects; Protein Conformation - drug effects; SODIO; SODIUM; Sodium - chemistry; SPECTROMETRIE; Spectrophotometry, Infrared; SPECTROSCOPIE INFRAROUGE; Spectroscopy : techniques and spectras; Structure-Activity Relationship; ZINC
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi00100a010

We have examined the influence of monovalent and divalent cations on the secondary structure of bovine alpha-lactalbumin at neutral pH using Fourier-transform infrared spectroscopy. Our present studies are based on previously reported amide I' component band assignments for this protein [Prestrelski, S. J., Byler, D. M., and Thompson, M. P. (1991) Int. J. Pept. Protein Res. 37, 508-512]. The results indicate that upon dissolution, alpha-lactalbumin undergoes a small, but significant, time-dependent conformational change, regardless of the ions present. Additionally, these studies provide the first quantitative measure of the well-known secondary structural change which accompanies calcium binding. Results indicate that removal of Ca2+ from holo alpha-lactalbumin results in local unfolding of the Ca2+-binding loop; the spectra indicate that approximately 16% of the backbone chain changes from a rigid coordination complex to an unordered loop. We have also examined the effects of binding of several other metal ions. Our studies have revealed that binding of Mn2+ to apo alpha-lactalbumin (Ca2+-free), while inducing a small, but significant, conformational change, does not cause the alpha-lactalbumin backbone conformation to change to that of the holo (Ca2+-bound) form as characterized by infrared spectroscopy. Similar changes to those induced by Mn2+ are observed upon binding of Na+ to apo alpha-lactalbumin, and furthermore, even at very high concentrations (0.2 M), Na+ does not stabilize a structure similar to the holo form. Binding of Zn2+ to the apo form of alpha-lactalbumin does not result in significant backbone conformational changes, suggesting a rigid Zn2+-binding site. Further, as characterized by infrared spectroscopy, binding of Zn2+ to holo alpha-lactalbumin does not induce a reversion to an apolike conformer